1. Field of the Invention
The present invention relates to X-ray screening systems for airport luggage and the like; and, more particularly to screening systems that utilize radiation transmitted through and scattered from an object under inspection to detect weapons, narcotics, explosives or other contraband.
2. Description of the Prior Art
X-ray inspection systems that use transmitted radiation have conventionally been employed to detect the shape of high-Z material (Z refers to atomic number) such as steel. The principle objective of these systems is the detection of weapons, such as guns, knives, bombs and the like. A major problem with X-ray inspection systems is the inability thereof to accurately recognize and detect explosives and narcotics made up of low-Z materials. Recently dual energy X-ray systems have been used to improve the detection of low-Z material. Such systems measure the different attenuation that high and low transmitted energy X-rays experience as a result of passage through any material. This principal has allowed the identification of virtually any material so long as the material is not covered by a different Z material. In order to overcome the material overlaying problem, it has been proposed that X-ray transmission be effected from different directions using two X-ray sources, or that the object be scanned from all sides and the results be evaluated with computer tomography.
Another approach for identifying low-Z material involves detecting the Compton scattered radiation along with the transmitted radiation. Low-Z material such as explosives and narcotics generates more scattered radiation than high-Z material like iron. This scattered radiation differential provides a basis for distinguishing between low-Z and high-Z material in instances where the low-Z material is concealed behind high-Z material.
Among the more troublesome problems with X-ray transmission and Compton scatter images are their poor resolution and high noise content. The causes of these problems are traced to: a) the relatively poor light collection method used in converting X-ray photons to light photons; and b) photon integration. Poor light collection presents a problem because it requires use of slow (long persistence) phosphor type X-ray detectors. Such detectors oftentimes create blurred images owing to the slow response time of the excited phosphor. The use of photon integration in conventional signal generation produces noisy images, particularly in cases where the transmitted or backscattered X-ray rates are relatively small. For example, U.S. Pat. No. 5,260,982 to Fujii et al. discloses a scattered radiation imaging apparatus. The Fujii et al. apparatus employs long persistence phosphor type X-ray detectors and photon integration yielding relatively low resolution, high noise images.
Employing relatively fast (very short persistence) phosphors such as Gd2SiO5 or Y2O2Si, or an organic plastic scintillator (loaded or unloaded with lead or tin), for faster X-ray detection efficiency, would significantly improve image resolution. When coupled with photon-counting electronics to reduce noise, overall image quality would be significantly improved. These improvements would yield a sharper image more capable of recognizing bombs, currency, narcotics and other contraband shapes and accompaniments.
The present invention provides a system and method for X-ray inspection of an object to detect weapons, narcotics, explosives or other contraband.
When an X-ray photon is absorbed by the scintillator, the scintillator generates photons in the visible part of the spectrum. These photons travel down the scintillator and enter a photon detector, such as a photomultiplier, that is coupled to the scintillator. The photomultiplier effectively converts the photons to electrons that can be processed by electronics for image generation. For the backscatter detector, the X-ray signals will be processed in the counting mode where individual X-rays are counted to generate the Compton backscatter image. For the transmitted beam image, the number of X-ray photons that enter the scintillator can vary over a wide dynamic range that is dependent on the object under inspection. In the case where there is no object or a very weak absorbing object the X-ray rate on the scintillator can be so high that counting individual X-rays is not possible. On the other extreme, for a highly attenuating object the X-ray rate would be very low or even zero. To accommodate this wide range of X-ray rates, the transmission detector system operates in a combined mode comprised of photon counting and photon integrating modes, where the mode is dynamically selected depending on the X-ray rate. This optimized method of collecting X-ray signals yields a superior image, as opposed to using only photon counting or photon integration.
The spatial resolution in the horizontal plane is accomplished via a pencil beam scanning across the inspection tunnel while a conveyor moves the object through the inspection tunnel. As only one line through the object is excited by the pencil beam at any time, the radiation captured by any scintillation detector is independent from the locus of the scintillation material that is actually hit by an X-ray photon, and must originate from this pencil line. The location of the pencil beam within the object image can be derived from the conveyor moving the object and the rotating disk with apertures that generate the pencil beam. It is possible to generate a direct luminescent image of an object with the transmission detector and an enhanced low Z image from the backscatter detectors and display them on separate monitors.
Preferably, the images produced by the transmission detector and backscatter detector are displayed as adjacent windows of the same monitor. Signal information from the transmission detector is used to correct for attenuation effects in the backscatter images, thereby avoiding artifacts in the low Z images produced by attenuation due to high Z objects. Conversely, signal information from the backscatter detectors can be used to correct for scatter effects in the transmission image, thereby avoiding artifacts in the high Z image produced by scattering attenuation due to low Z objects. By means of these corrections a greater fraction of the image on the high Z window display is derived from absorption effects of high Z objects, and a greater fraction of the image on the low Z window display is derived from scattering effects of low Z objects. Accordingly, the images displayed by the high Z and low Z windows are more distinct.
Tomographic information can optionally be obtained by using additional Compton backscatter detectors. Backscattered X-rays originating from elements close to the bottom of the object hit mainly the scintillator next to the entrance slit, while backscatter from elements further up the pencil beam hit all backscatter scintillation detectors nearly equally. Photon collection efficiency is improved and real-time image noise is reduced, when compared to collimation methods that limit angular admittance of photons. The tomographic zones can be displayed in windowed sections on a single monitor or on separate monitors.
With at least one additional detector overlaying the extant transmission detector, or by dividing the extant transmission detector into low energy and high energy components, dual energy information can be obtained. This information can be displayed as a dual energy image, which is color coded to designate the atomic number of an object under inspection. A single energy image yields only object radiographic density information, as contrasted to a dual energy image, which yields radiographic density and atomic number, Z, of the object under inspection. Combining the information from the backscatter data and dual energy data can further enhance discrimination of different materials and aid in the separation of overlaying materials of different atomic number Z.
By employing relatively fast scintillators for faster X-ray detection efficiency, the present invention significantly improves image resolution. Detector design is improved by the use of optically adiabatic scintillators. The system switches between photon-counting and photon integration modes to reduce noise and significantly increase overall image quality. In addition, the system automatically adjusts belt speed (i) to allow rapid entrance into the inspection zone, (ii) slow traverse through the inspection zone to prolong residence therein of articles appointed for inspection, and (iii) allow rapid exit from the inspection zone. This automatic belt speed adjustment feature affords increased resolution and reduced noise with minimum speed penalty. Advantageously, the system provides a sharper image that is more capable of recognizing bombs, currency, narcotics and other contraband shapes and accompaniments.